April 23rd Severe Weather Thread

[Quincy]

As we move into the final third of April, there are signs of more severe threats targeting the Plains after what has been a very slow start to the severe weather season.

Models have come into pretty good agreement on Wednesday. The SPC outlined an area from north-central Texas to southeastern Nebraska as being under the threat for severe thunderstorms (Day 4/Wednesday). As a shortwave swings east from the Pacific Northwest into some modest ridging over the Mississippi Valley, the stage is set.

By Wednesday afternoon, the models show a surface low developing over the west-central Plains. While the moisture influx has come into question, the latest 12z GFS now advects 60F dew-points all the way to southern Nebraska. The Euro has been fairly steady with dew-points in the upper 50s to lower 60s. Kinematic support is essentially a given with a low-level jet increasing to 40-60kt at 850mb by late afternoon and early evening. Instability and more-so capping were bigger question marks. There is strong support for a corridor of moderate instability from north Texas up into central Kansas ahead of the surface low. SBCAPE is progged in the range of 2000-2500 J/kg over much of the area. The 12z GFS has also trended more impressive in terms of eroding much of the cap by mid-afternoon.

The focus for supercells will largely be just ahead of a dryline. The parameters would suggest a threat for very large hail and damaging winds. While tornadoes are also possible, it's a bit too soon to say how widespread or intense that threat could be.

While some activity is possible on Thursday, the greatest severe threat targets Wednesday. Refer to the medium/long-range threat for further discussion on what could be a continued active pattern to close out April as a deeper trough may dig into the western CONUS.

[Chicago Storm]

Wednesday has always looked good if you're looking to chase structure and not tornadoes...and it continues to look that way.

[Stebo]

Wednesday has always looked good if you're looking to chase structure and not tornadoes...and it continues to look that way.

Yeah Wednesday certainly could have some big hailers along the dryline.

[andyhb]

This discussion is for D4 system, the medium/long range discussion is for what comes after.

[Ground Scouring]

The GFS, including today's 18Z run, has consistently forecast soundings along the dryline at 00Z Thursday that are more than supportive of rotating supercells in an area stretching from Woodward, OK, to SE of Dodge City, KS. Dewpoints keep going up with each run, which now shows widespread dewpoints of between 60-65°F across almost all of OK and parts of s-ctrl KS. The GFS has also consistently shown the dryline near the OK/KS border lighting up with convection as early as just before 00Z Thursday. With backed low-level winds all along the dryline, helicities will be maximized with a strong/strengthening H85 low-level jet toward late afternoon. Combined with the risk for substantial hail I think a MDT Risk (perhaps as early as Day 2, but more likely Day 1) might eventually be posted for at least part of the area I mentioned.

[Chicago Storm]

The GFS, including today's 18Z run, has consistently forecast soundings along the dryline at 00Z Thursday that are more than supportive of rotating supercells in an area stretching from Woodward, OK, to SE of Dodge City, KS. Dewpoints keep going up with each run, which now shows widespread dewpoints of between 60-65°F across almost all of OK and parts of s-ctrl KS. The GFS has also consistently shown the dryline near the OK/KS border lighting up with convection as early as just before 00Z Thursday. With backed low-level winds all along the dryline, helicities will be maximized with a strong/strengthening H85 low-level jet toward late afternoon. Combined with the risk for substantial hail I think a MDT Risk (perhaps as early as Day 2, but more likely Day 1) might eventually be posted for at least part of the area I mentioned.

Unfortunately what the GFS shows DP wise is pointless to mention.

 

It's way off even even in the short term...18z run 6hr forecast for 0z this evening is terrible. NAM is much closer to reality.

 

Guidance as a whole has over-done DP's every event this month. Today was no different, and Wed won't be either.

[brettjrob]

Unfortunately what the GFS shows DP wise is pointless to mention.

 

It's way off even even in the short term...18z run 6hr forecast for 0z this evening is terrible. NAM is much closer to reality.

 

Guidance as a whole has over-done DP's every event this month. Today was no different, and Wed won't be either.

 

Agreed for the most part. Dew points will likely verify 54-58 F along most of the dryline. That's not a show-stopper for areas along 100 W (~2000 ft. elevation), but it's sure not great. It would take a Campo-like fluke or mesoscale accident to get a decent tornado. Some nice supercells still look likely, as has been covered.

 

If dew points were going to be 62-65 F for the same setup, we'd quite possibly be talking about a regional outbreak. Fail.

[Stebo]

Unfortunately what the GFS shows DP wise is pointless to mention.

 

It's way off even even in the short term...18z run 6hr forecast for 0z this evening is terrible. NAM is much closer to reality.

 

Guidance as a whole has over-done DP's every event this month. Today was no different, and Wed won't be either.

 

GFS showed 6hr forecast of 60-65 dews for most of Central/Southern TX at 00 and currently that is what there is. Elsewhere it shows 55-60 dews and those are verifying as well... The NAM is actually too high with respect to dews compared to reality, so it is exactly the opposite of what you are saying. Oh and LOL at using the NAM beyond 48 hours, especially when it can't forecast the first 12 hours correctly.

 

Edit: Actually that is the 12z GFS that has the dews correctly placed, neither 18z model is correct for right now.

[tornadotony]

GFS showed 6hr forecast of 60-65 dews for most of Central/Southern TX at 00 and currently that is what there is. Elsewhere it shows 55-60 dews and those are verifying as well... The NAM is actually too high with respect to dews compared to reality, so it is exactly the opposite of what you are saying.

 

Edit: Actually that is the 12z GFS that has the dews correctly placed, neither 18z model is correct for right now.

That's because of this little thing called model spinup.  Trying to verify the NAM or especially the GFS 6h out is pretty foolish.

[Chicago Storm]

GFS showed 6hr forecast of 60-65 dews for most of Central/Southern TX at 00 and currently that is what there is. Elsewhere it shows 55-60 dews and those are verifying as well... The NAM is actually too high with respect to dews compared to reality, so it is exactly the opposite of what you are saying.

 

Edit: Actually that is the 12z GFS that has the dews correctly placed, neither 18z model is correct for right now.

C'mon stebo...this is easy.

 

[Stebo]

C'mon stebo...this is easy.

 

 

You do know that one on the left is a model generated dewpoint map right? How about looking at the observations instead... Plus if you read my edit, I said neither are correct.

[Chicago Storm]

You do know that one on the left is a model generated dewpoint map right? How about looking at the observations instead...

Yes, I did cross-check....which is why I went with it.

[andyhb]

Unfortunately what the GFS shows DP wise is pointless to mention.

 

It's way off even even in the short term...18z run 6hr forecast for 0z this evening is terrible. NAM is much closer to reality.

 

Guidance as a whole has over-done DP's every event this month. Today was no different, and Wed won't be either.

 

What events?

 

Besides 4/3, which was also severely mesoscale affected, there hasn't been much to develop any sort of conclusion in that department.

[Chicago Storm]

What events?

 

Besides 4/3, which was also severely mesoscale affected, there hasn't been much to develop any sort of conclusion in that department.

The 4/1-43 event, 4/12-13 and now today. Obviously not a huge sample size...but every event.

 

And I know I'm not the only one that has noticed this.

[Chicago Storm]

That's because of this little thing called model spinup.  Trying to verify the NAM or especially the GFS 6h out is pretty foolish.

Any of the last few runs aren't close...Had to choose one as an example though obviously.

[andyhb]

I'd like to see some pieces of guidance from before 4/13 and then compare them to observations the day of, there were mid/upper 60s dews that day in central TX and SE OK. The weaker low level shear was more the cause of the lack of tornadoes that day, especially as they moved into the deeper moisture towards the I-35 corridor (and by that time things were beginning to become capped). I also think the anvil level SR winds were a major issue since the anvils kept blowing off to the ENE/NE on satellite/radar images.

[tornadotony]

Any of the last few runs aren't close...Had to choose one as an example though obviously.

You say that like it's a surprise...it's not.

[Chicago Storm]

I'd like to see some pieces of guidance from before 4/13 and then compare them to observations the day of, there were mid/upper 60s dews that day in central TX and SE OK.

The issue with that event was on 4/12.

 

I don't have anything saved, but IIRC Brett posted something on FB from 4/12.

[brettjrob]

I'd like to see some pieces of guidance from before 4/13 and then compare them to observations the day of, there were mid/upper 60s dews that day in central TX and SE OK. The weaker low level shear was more the cause of the lack of tornadoes that day, especially as they moved into the deeper moisture towards the I-35 corridor (and by that time things were beginning to become capped). I also think the anvil level SR winds were a major issue since the anvils kept blowing off to the ENE/NE on satellite/radar images.

 

Well, I think there's a substantial difference between NE TX and where the dryline sets up this coming Wednesday. While I don't have time to dig up a ton of examples right now, would anyone dispute that over the past few years, moisture has almost always verified worse than forecast over the Panhandles, W OK, and W KS? The magnitude of deficit varies from one event to another, but I can't remember a single dryline event since 2011 where moisture hasn't mixed out in that region by late afternoon, resulting in T/Td spreads considerably larger than guidance like the GFS/NAM/ECMWF indicated.

 

If there's any silver lining, the downstream toughiness and associated surface cyclone off New England has trended a bit more eastward over the past couple days. Still, the continental synoptic pattern is not one I'd associate with good moisture return in mid-April, and any vigorous mixing is probably going to do serious damage. We'll have to wait until Wednesday to say for sure, but I'd be really, really surprised if more than 1-2 isolated ASOS sites reads Td >=60 F at 5pm in the corridor ahead of the dryline. That's not to say this isn't worth watching, though. It's almost certainly going to be the most significant event in the affected region thus far in 2014.

[andyhb]

Well, I think there's a substantial difference between NE TX and where the dryline sets up this coming Wednesday.

 

Obviously.

 

I was posting on a case by case basis.

[tornadotony]

The BFD is not going to Wednesday.  It's going to be Sunday-Monday.

[Quincy]

While still not overly impressive, the 00z Euro has bumped up both 2m temps and dew-points 1-3F higher than the previous run for Wednesday PM. Euro verbatim looks like ~57F dews just ahead of the dryline, but a larger area of 58-62F dews across much of Kansas and even into northeastern Nebraska.

[Chinook]

The BFD is not going to Wednesday.  It's going to be Sunday-Monday.

You should explain that. I think BFD means Bradford, Pennsylvania.

[AgeeWx]

You should explain that. I think BFD means Bradford, Pennsylvania.

 

The 'Big ****in' Deal' is going to be this weekend, not Wednesday. We've been so deprived of severe events this season that we're more focused on the next, next event rather than the upcoming one.

[Ed Lizard]

There is actually some rotation (at what level, don't know, free NWS radar) on the cells well West of I-35.

[AgeeWx]

Moisture return for Wednesday is going to be largely dependent on when and where the anti-cyclone sets up in the Gulf. 00Z GFS (compared to the 12Z) had a much more favorable location, off the coast of LA, for moisture return ahead of the system.

[David Reimer]

Awesome AFD from Simpson in Amarillo regarding Wednesday's event. 

 

WEDNESDAY...
THOSE WARM OVERNIGHT LOWS WILL LEAD TO A WARM WEDNESDAY. BY EARLY IN
THE AFTERNOON, A DRYLINE WILL BE COMING TOGETHER NICELY AND SET THE
STAGE FOR A CLASSIC 2-PRONGED SPRING DAY OF POTENTIAL SVR IN THE
EAST AND VERY DRY AND WINDY CONDITIONS IN THE WEST. GOING FORECAST
LOOKS GREAT AND DID NOT CHANGE POP/WX STRUCTURE, WITH 50 POPS IN THE
EAST AND 0 POPS IN THE WEST.

CHASER/WX NERD VERSION...
CONSISTENCY IN THE MODELS HAS BEEN EXCELLENT WITH THIS SYSTEM...BUT
THE ONLY DIFFERENCES INVOLVE TIMING/STRENGTH OF SPEED MAXES WHICH
WILL PLAY A PART IN THE EVOLUTION OF SVR WX. THE NAM STILL HAS THE
STRONGEST MID AND UPR FLOW...BUT THE DIFFERENCES BETWEEN MODELS ARE
GETTING SMALLER. FORECAST SOUNDINGS STILL INDICATE WEAK CAPPING AND
ALL SIGNS STILL POINT TO CONVECTIVE INITIATION BY EARLY TO MID
AFTERNOON JUST OFF THE CAPROCK...PROBABLY ALONG AND JUST EAST OF AN
AMARILLO TO GUYMON LINE. FAIRLY STRAIGHT AFTERNOON HODOGRAPHS WITH
BULK SHEAR VALUES OF 30-40KTS IN THE PRESENCE OF MODEST INSTABILITY
(1500-2000 J/KG) CONTINUE TO INDICATE SUPERCELL (SOME SPLITTING)
STORM MODE, AT LEAST INITIALLY. GIVEN WEAK CAPPING AND MODEST
FORCING...STORM COVERAGE MAY BE RELATIVELY WIDESPREAD...MEANING
STORM INTERACTIONS COULD RESULT IN RAPID UPSCALE GROWTH TO A BROKEN
LINE BY MID TO LATE AFTERNOON. LARGE HAIL TO THE SIZE OF BASEBALLS
WILL BE THE MAIN THREAT...ESPECIALLY IN INITIAL ISOLATED CONVECTION.
ANY LINEAR ORGANIZATION WOULD ALSO POSE A WIND THREAT AS COLD POOL
GENERATION WILL BE POSSIBLE GIVEN THE SUB CLOUD DRY LAYER. IN
ADDITION...0-1KM SHEAR VALUES OF 20-30KTS MAY FAVOR A WELL BALANCED,
LONG-LIVED GUST FRONT.

UNFORTUNATELY FOR CHASERS BUT FORTUNATELY FOR EVERYONE ELSE...THE
TORNADO THREAT LOOKS LOW FOR SEVERAL REASONS: 1) WEAK LOW LEVEL
TURNING OF WINDS WITH HEIGHT, 2) HIGH LCLS ABOVE 2000M, AND 3)
POTENTIAL STORM INTERACTION RESULTING IN UPSCALE GROWTH OF STORMS
INTO CLUSTERS OR A LINE. AS FAR AS THE LCLS...EVEN THE MODEL WITH THE
MOISTEST LOW LVL SOLUTION, THE SREF, SHOWS ABOUT A 70 PERCENT CHANCE
THAT LCLS STAY GREATER THAN 1500M EVERYWHERE THROUGH 03Z THU. ONE
COULD ARGUE THAT THERE IS A SMALL TORNADO WINDOW WITH ANY SUPERCELLS
THAT REMAIN INTO THE MID TO LATE EVENING HOURS AS CLOUD BASES LOWER
AND INITIAL BOUNDARY LAYER DECOUPLING PROMOTES INCREASING 0-1 SRH
VALUES. HOWEVER, BY THAT POINT FCST SOUNDINGS ALL INDICATE THE NEAR
SFC INVERSION STRENGTHENING TO A LEVEL THAT WILL QUICKLY PRECLUDE SFC
BASED CONVECTION. WE DO LIKE TO ALWAYS INCLUDE THE CAVEAT THAT ANY
LOCALIZED BAROCLINIC ZONES (IE BOUNDARIES) COULD AUGMENT THE LOW LVL
ENVIRONMENT SUFFICIENTLY TO ALLOW SUPERCELLS TO BECOME TORNADIC, AND
THOUGH UNLIKELY, WEAK TORNADOES ARE NOT UNHEARD OF WITH LCL HEIGHTS
ABOVE 2000M.

ALL SAID...HAIL IS THE GREATEST THREAT FOLLOWED BY WIND WITH THE
BEST SVR STORM WINDOW EAST OF AN AMARILLO TO GUYMON LINE FROM 2P-9P
WED.

[Chinook]

I wonder if tomorrow there will be the first storm reports west of (or near) Dodge City for the whole year so far.

[nrgjeff]

JIT moisture is a problem Wednesday. Obviously it is not a travel day. However if I still lived there, it is equally obviously a local chase day. Most likely outcome is hail but excellent supercell structure. With gas at nearly $4 I'd not chase just for structure, but the chance of a quick tornado would make a local chase worth it. West-central Oklahoma looks like a reasonable starting point. Tomorrow one will need to carefully nowcast the intersection of a subtle differential heating boundary and the dry line.

[Chinook]

the 18z 4-km NAM suggests a strong squall line, perhaps a bow echo, at the KS/NE border, tomorrow, 0z to 3z (7pm-10pm). This actually has a cool-looking double-bow echo shape here.

 

The 18z NAM forecast sounding for 21z tomorrow shows about 1700 J/kg of CAPE, with a high helicity value (300-400m2/s2) in north central Kansas, due to 850mb wind of about 50kt. 500mb wind is less than 50kt.